semicolon inside and outside Promise object

Question

When I run the following code:

my $timer = Promise.in(2);
my $after = $timer.then({ say "2 seconds are over!"; 'result' });
say $after.result;  # 2 seconds are over
                    # result

I get

2 seconds are over!
result

What is the role of ; inside the then and why if I write

say "2 seconds are over!"; 'result';

do I get the following error?

WARNINGS:
Useless use of constant string "result" in sink context (line 1)
2 seconds are over!

and not :

2 seconds are over!
result

like the first example?

Answer 1

'result' is the last statement of the block { say "2 seconds are over!"; 'result' }. In Perl languages, semicolons (not newlines) determine the ends of most statements.

In this code:

my $timer = Promise.in(2);
my $after = $timer.then({ say "2 seconds are over!"; 'result' }); # 'result' is what the block returns
say $after.result;  # 2 seconds are over (printed by the say statement)
                    # result ('result' of the block that is stored in $after)

The second line could be rewritten thus:

my $after = $timer.then( {
                          say "2 seconds are over!";
                          'result'
                         }
); # 'result' is what the block returns

That semicolon simply ends the statement say "2 seconds are over!".

Outside of a block, this line

say "2 seconds are over!"; 'result';

is really two statements:

say "2 seconds are over!";  # normal statement
'result';                   # useless statement in this context (hence the warning)

Putting multiple statements in one line rarely changes how they behave:

my $timer = Promise.in(2); my $after = $timer.then({ say "2 seconds are over!"; 'result' }); say $after.result; # Still behaves the same as the original code. ... Do not edit. This is intentionally crammed into one line!